Chapter 1 of the book "Analog Signal Processing and Instrumentation", by Arie Arbel, Cambridge University Press, 1980 discusses both voltage-mode and current-mode operational amplifiers. FIG. 1 attached hereto shows a typical voltage-mode feedback stabilized amplifier comprising a basic amplifier and a feedback network consisting of resistors Rf and Ro operating in the voltage mode. Its transfer-function approaches vo/vg=(1+Rf/Ro), if the voltage gain Av of the basic amplifier approaches infinity. Hence, gain accuracy depends upon the capability to obtain an accurate ratio between two linear, passive impedances, usually resistors and to maintain this ratio over the range of operating temperatures. The presence of resistors complicates the manufacture of integrated circuits. However, resistors are linear components and lead to desired linear transfer functions from operational amplifiers.
FIG. 2 shows a recent version of current mode single ended feedback stabilized operational amplifier (CMA). By "single ended" a single input and a single output is meant. This distinguishes from a differential amplifier having two outputs of opposite phase. The CMA employs a current mode output stage (COS) such as an FCS (floating current source) or a CCII (class II current conveyor) as the output stage. The FCS is a novel building block employed in analog current mode circuit design, described in the paper "Output Stage for Current-Mode Feedback Amplifiers, Theory and Application", Arie F. Arbel and Lavy Goldminz, Analog Integrated Circuits and Signal Processing, 2, (3) (1992), pp. 243-255. Both the FCS and the CCII are characterized by the fact that the current Iout entering the first current output terminal equals the current Iout flowing out of the second current output terminal. Both are novel building blocks in analog current-mode circuit design which are described in the paper entitled "Output Stage for Current-Mode Feedback Amplifiers, Theory and Application" by A. Arbel at al., Analog Integrated Circuits and Processing 2, (3) 1992, pp 243-255. The basic amplifier as an example is shown in FIG. 2 as a transimpedance amplifier ZT driving a COS, whose two outputs are connected to the output terminal and to the feedback network, respectively. The output terminal is connected to ground through a load.
The voltage or current transfer function, respectively, for infinite basic amplifier gain, is ( for both kinds of feedback amplifiers): (1+Rf/Ro). Note, that the relationship between FIGS. 1 and 2 is governed by the law of duality: The input impedance of the ideal basic voltage amplifier in FIG. 1 is infinite, whereas that of the ideal basic current amplifier in FIG. 2, consisting of the transimpedance amplifier ZT in series with the transconductance of the FCS, is zero. The input current of the ideal basic voltage amplifier (i.e., voltage gain Av=infinity) equals zero, whereas the voltage at the input terminal X of the ideal basic current amplifier (i.e, currant gain Ai=infinity), is zero as well. In FIG. 1, the two feedback resistors Rf and Ro are in series, whereas in FIG. 2 they are in parallel (dc wise) as far as dc voltages are concerned. Resistor Rf will be identified as the feedback resistor and resistor Ro as the gain setting resistor.